Abnormal condition detection device for corona discharger in electrophotographic copying machine

Abstract

An electrophotographic copying machine including some corona dischargers each having a high voltage transformer and an abnormal condition detection device. The abnormal condition detection device includes a detection unit for detecting an output level at a secondary winding of the transformer, an input unit for inputting an input voltage to a primary winding of the transformer. A comparator is provided for comparing the output level on the detection unit with the input level on the input unit, and outputting a detection signal. A control circuit of the copying machine is responsive to the level of the detection signal for controlling the operation of the copying machine.

What is claimed is: 
     
       1. A device for detecting an abnormal condition in a corona discharger of an electrophotographic copying machine, said corona discharger including a corona electrode spaced from a photoconductive surface, and a transformer with a primary winding and a secondary winding coupled to the corona electrode for applying a charging voltage thereto, said device comprising: (a) comparator means having first and second input terminals;   (b) means for applying a reference voltage from said primary winding of said transformer to the first input terminal of the comparator means;   (c) resistor means in circuit with the second input terminal of said comparator means for converting currents passing through the resistor means to a control voltage at the second input terminal;   (d) means for applying a feedback current from the secondary winding of said transformer through said resistor means, said feedback current creating a control voltage higher than said reference voltage during normal operating conditions of the corona electrode, said feedback current becoming essentially zero when an open circuit is formed between the corona electrode and ground, thus providing a control voltage of essentially zero at the second input terminal of said comparator;   (e) constant current control means in circuit with both said secondary winding and said resistor means for applying a constant current through said resistor means of a limited value which will create a control voltage which is less than said reference voltage, said constant current control means generating a current of said limited value responsive to a low charging voltage output by the secondary winding on said corona electrode, said low charging voltage being of a value indicative of a short circuit between the corona electrode and ground; and   (f) means for generating an abnormal condition signal indicative of either a short or open circuit between said corona electrode and ground whenever the control voltage at the second input of the comparator means is less than the reference voltage at the first input of the comparator means.   
     
     
       2. The device of claim 1, futher including display means responsive to the abnormal condition signal providing a visual indication of the abnormal condition. 
     
     
       3. A system for detecting abnormal conditions in a plurality of corona dischargers of an electrophotocopying machine and determining which corona discharger has an abnormal condition associated therewith, each said corona discharger including a corona electrode spaced from a photoconductive surface, a transformer with a primary winding and a secondary winding coupled to the corona electrode for applying a charging voltage thereto, said device comprising: (a) a detection device for each corona discharger including, 1. comparator means having first and second input terminals,   2. means for applying a reference voltage from said primary winding of said transformer to the first input terminal of the comparator means,   3. resistor means in circuit with the second input terminal of said comparator means for converting currents passing through the resistor means to a control voltage at the second input terminal,   4. means for applying a feedback current from the secondary winding of said transformer through said resistor means, said feedback current creating a control voltage higher than said reference voltage during normal operating conditions of the corona electrode, said feedback current becoming essentially zero when an open circuit is formed between the corona electrode and ground, thus providing a control voltage of essentially zero at the second input terminal of said comparator,     
     
     
       5. constant current control means in circuit with both said secondary winding and said resistor means for applying a constant current through said resistor means of a limited value which will create a control voltage which is less than said reference voltage, said constant current control means generating a current of said limited value responsive to a low charging voltage output by the secondary winding on said corona electrode, said low charging voltage being of a value indicative of a short circuit between the corona electrode and ground; and 6. means for generating an abnormal condition signal indicative of either a short or open circuit between said corona electrode and ground whenever the control voltage at the second input of the comparator means is less than the reference voltage at the first input of the comparator means; and   (b) means for determining which of said detection devices generates an abnormal condition signal.   
     
     
       4. The system of claim 3, wherein said means for determining includes display means for visually displaying which corona discharger is experiencing an abnormal condition.